The internal volume of the current Gas Pressure Sensor is approximately 0.8 mL.
The internal volume of the older style (pre-2005) Pressure Sensor is about 1 mL, but the exact answer depends on the length of tubing connected. Here is a way to test the volume quickly:
1) Set up the sensor with an interface and program so that the pressure is being read live as you watch the screen.
2a) (Pressure Sensor or Biology Gas Pressure Sensor) Connect the syringe to the sensor. Open the valve so that outside air can come into the syringe. Move the piston down so the volume reads zero. Close the valve.
2b) (Gas Pressure Sensor) Move the piston on the syringe so that the volume reads zero. Thread the syringe onto the sensor.
3) The pressure should be close to one atmosphere. Note the exact reading.
4) Pull out the piston slowly, noting the pressure reading. When the reading drops to half of its original reading, you have doubled the volume. Hence that new piston reading is the volume of the pressure sensor and the tube up to the syringe.